Pixar shorts have always been test beds for techniques and technologies that show up in their feature films in the years afterward.

Indeed. And rumour is they've stopped doing them now.

On the VFX side of the industry we were often jealous of Pixar and Dreamworks being able to do shorts, in terms of having the scope and ability to test out new tech / pipelines.

They're still doing them, but they may not be featured before pictures anymore. They've found different distribution channels (YouTube, Disney+, Disney Channel, etc).

Any idea why they might have stopped?

Yeah, but then you realised why they were doing them, because they were using their own in-house tools.

Pixar was the last holdout for maya, they only converted in the last few years

Are you saying that Pixar is using Maya now? For which parts of the workflow?

Creativity Inc. is very worth reading. A guy has a dream and spends his life inventing things that bring his dream to reality.